I have made a mold from pop and am able to pour a bit with a white belly and a different color back also, it is fairly easy to do here are the steps:
1 make a mold using a knife like del said insert it into a bait and mold one side leaving the knife in the bait.
2 let this one set all the way up also remember to put in your pouring spouts, one at the nose of the bait and one at the back of the bait.
( the reason for two spouts is if you want a multi color bait plug the nose spout with plastic plug and pour through the back spout this way the bait fills the belly first and then so on with other colors, if just one color pour through the nose plugging the back.)
3 after the first side sets up pour the next side makeing sure mixture is a little thinner as not to bend the thin tail, remember that the knife has to stay in the bait, to make a insert for it when your pour.
4 after this sets seperate easily and seal NOW GO POUR THIS WORKS GREAT!!!!!
Hope this helps Jason